Many publications (cf. for example WO-94/21512, WO-97/12118, WO-91/15446 and U.S. Pat. No. 4,833,170) describe processes for the conversion of (gaseous) hydrocarbon feed, such as methane, natural gas and/or associated gas, into liquid products, especially methanol and liquid hydrocarbons, particularly paraffinic hydrocarbons. Such conversion processes may be operated at remote locations (e.g. in desserts, tropical rain-forests) and/or offshore locations, where no direct use of the gas is possible, due to the absence of large populations and industries. Transportation of the gas to populated and industrial areas, e.g. through a pipeline or in the form of liquefied natural gas, requires extremely high capital expenditure or is simply not practical. This holds even more in the case of relatively small gas production fields and/or relatively small gas production rates. Re-injection of gas into the production field will add to the costs of the oil production, and may, in the case of associated gas, result in undesired effects on the crude oil production. Burning of associated gas has become undesired in view of depletion of hydrocarbon sources and air pollution.
Gas found together with crude oil is known as associated gas, whereas gas found separate from crude oil is known as non-associated gas. Associated gas may be found as “solution gas” dissolved within the crude oil, and/or as “gas cap gas” adjacent to the main layer of crude oil. Associated gas is usually much richer in the larger hydrocarbon molecules (ethane, propane, butane) than non-associated gas.
Especially in view of the fact that the above-mentioned conversion processes may be operated at remote locations or at locations where limited space is available there is an incentive to place special emphasis on such factors as energy and cost efficiency, compactness and complexity of the process or the plant in which the process is carried out. From the references given above, however, no optimally integrated, efficient, low-cost process scheme is available.
WO-98/01514 discloses a process in which gaseous hydrocarbon feed is converted with air into syngas which, in turn, is converted into liquid hydrocarbon product in a Fischer-Tropsch synthesis step. A substantial amount of the heat generated in the process is recovered and re-used in the process. Further, an off-gas mixture which is co-produced in the Fischer-Tropsch synthesis is used to fuel a gas turbine which, in turn, is used to power the compressor needed for compressing the air used in the process. The off-gas mixture in question comprises unconverted syngas, methane by-product from the Fischer-Tropsch synthesis, and nitrogen originating from the air used. The use of air as the oxidant in the conversion of the gaseous hydrocarbon obviates the need of a production unit of an oxygen rich oxidant. However, the nitrogen present in the air acts in the process as a diluent gas, necessitating handling larger quantities of gas at a higher total pressure, which requires more compression capacity.
A further disadvantageous aspect of the use of air is that the said off-gas mixture is diluted with nitrogen which causes that it has a low heating value. The heating value is especially low when the syngas production and the Fischer-Tropsch synthesis are operated efficiently, so that the content of combustible materials in the off-gas mixture is further decreased. In the light of the process of WO-98/01514 this will represent a problem when the heating value is so low that the off-gas mixture is unsuitable for use as a gas turbine fuel.